Mechanical systems, such as internal combustion engines and automatic transmissions, typically include a lubrication pump to provide lubricating oil, under pressure, to many of the moving components and/or subsystems of the mechanical systems. In most cases, the lubrication pump is driven by a rotating component of the mechanical system and thus the operating speed and output of the pump varies with the operating speed of the mechanical system. The lubrication requirements of the mechanical system do not directly correspond to the operating speed of the mechanical system.
To deal with these differences, prior art fixed displacement lubricating pumps were generally designed to operate effectively at a target speed and a maximum operating lubricant temperature resulting in an oversupply of lubricating oil at most mechanical system operating. A pressure relief valve was provided to return the surplus lubricating oil back into the pump inlet or oil sump to avoid over pressure conditions in the mechanical system. In some operating conditions such as low oil temperatures, the overproduction of pressurized lubricating oil can be 500% of the mechanical system's needs. The result is a significant amount of energy being used to pressurize the lubricating oil which is subsequently exhausted through the relief valve.
More recently, variable displacement vane pumps have been employed as lubrication oil pumps. Such pumps generally include a control ring, or other mechanism, which can be operated to alter the volumetric displacement of the pump and thus its output at an operating speed. Typically, a feedback mechanism is supplied with pressurized lubricating oil from the output of the pump to alter the displacement of the pump to operate and to avoid over pressure situations in the engine throughout the expected range of operating conditions of the mechanical system.
While such variable displacement pumps provide some improvements in energy efficiency over fixed displacement pumps, they still result in a significant energy loss as their displacement is controlled, directly or indirectly, by the output pressure of the pump which changes with the operating speed of the mechanical system, rather than with the changing requirements of the lubrication system. Accordingly, such variable displacement pumps must still be designed to provide oil pressures which meet the highest expected mechanical system requirements, despite operating temperatures and other variables, even when the mechanical system operating conditions normally do not necessitate such high requirements.
Another variable displacement pump control system is described within U.S. Pat. No. 7,018,178. The control system includes an electrical solenoid coupled to a variable displacement pump for varying the displacement of the pump during engine operation. While an electric solenoid may provide an additional degree of pump control, several disadvantages from its use exist. In particular, a solenoid requires a continuous supply of current to keep it active through operation of the pump. The use of the electrical power offsets the benefit of controlling the pump to minimize the amount of time where the pump provides excess lubricant flow. Furthermore, the maximum force capability of the solenoid is limited by the size of the electromagnet and the current applied thereto. For certain applications, the size of the electromagnet required to provide the desired force may be prohibitive for packaging the solenoid within an automotive environment. Accordingly, a need exists for an improved lubrication system capable of producing a desired lubricant flow while minimizing the energy required to do so.